3.15 > Rising sea levels impact differently on coastal areas and their inhabitants. Societies may take steps to protect themselves, but the costs can be substantial and ultimately exceed the benefits. The measures are classified as: [P] – Protective, [A] – Adaptive, and [R] – Retreat measures

An old saying for tomorrow:
Build a dyke or move away

Ever since first settling along the coast, human societies have had to come to terms with changing conditions and the threat of storms and floods. Over time they devel­oped ways of protecting themselves against the forces of nature. Today four distinct strategies are used, none of them are successful in the long term:

Adaptation of buildings and settlements (artificial dwelling hills, farms built on earth mounds, pile houses and other measures);

A culture of risk developed early on in Europe and parts of East Asia (Japan, China). Phases of retreat and adaptation until the Middle Ages were followed in more modern times by a strategy of defence; a strategy adopted in North America and other areas which were settled later. The effective protection of low-lying regions and coastal cities from flooding, land loss, water-logging and groundwater salinity is a both costly and technologically complex process. However, the example of the Netherlands shows that a small and affluent industrialized na­tion, when faced with a serious potential threat, is certainly capable of following the strategy of defence over the long term – after all, virtually two thirds of its country lies below the mean high-water mark. Germany also invests heavily in maintaining and protecting its much longer coastline with dykes and other structures. Each year the Netherlands and Germany together spend about 250 million euros on coastal defence. Although this amounts to only 0.01 per cent of the German and 0.05 per cent of the Netherlands’ gross national income, it should not be forgotten that these amounts are utilized for the maintenance and/or fortification of existing defence works. Much poorer coastal and small island states are not in a position to protect their coastlines on a similar scale. Confronted with rising sea levels they have the choice of either adapting or retreating. But even resettlement projects like that of the Carteret Islands, part of Papua New Guinea, which began in 2007, are costly. It is not yet possible to accurately assess the exact cost of evacuating 1700 people, but this will certainly amount to several million US dollars.

3.16 > Nations with the largest popula­tions and the highest proportions of population living in low-lying coastal areas. Countries with fewer than 100,000 inhabitants are not included. Also excluded are 15 small island states with a total of 423,000 inhabitants.

There are different strategies for combating and coping with the effects of rising sea levels. Whether a measure is used at a regional or local level depends mainly on the cost and the geological features of the area. In the Ganges-Brahmaputra delta region of Bangladesh for instance, heavy sea dykes would sink into the soft subsoil. Also, there is no money available to erect hundreds of kilo-metres of dykes. Such a project is likely to cost more than 20 billion euros – at least a hundred times more than the annual coastal defence costs of the Netherlands and Germany combined. The national economy of Bangladesh could not support anything like this amount. In other areas there are simply not enough building materials to protect the coast. Many coral islands do not have the sediment they need to hydraulically fill the coastline, or the space and building materials for dykes and sea walls. Even if enough cash were available, these islands would still be largely defenceless against the sea. The threat from rising sea levels is worsened by the fact that coralline limestone is being removed from the reefs and used to build hotel complexes.

It is impossible to foresee with any accuracy what the rising sea levels will mean for coastal and island nations and their defence in the 21st century, as this largely depends on the extent and speed of developments. If they rise by much more than 1 metre by 2100, then the dykes and protective structures in many places will no longer be high enough or stable enough to cope. New flood control systems will have to be built and inland drainage systems extensively upgraded. Experts anticipate that the annual costs of coastal protection in Germany could escalate to a billion euros – to protect assets behind the dykes worth 800 to 1000 billion euros. On a global scale the cost could be a thousand times greater. Although the costs of defence and adaptation would appear worthwhile to some nations in view of the substantial economic assets protected by the dykes, the poorer coastal areas will probably be lost or become inhabitable. The inhabitants will become climate refugees.

Presumably the industrialized countries are capable of holding back the sea for some time using expensive, complex coastal protection technology. But even there, this strategy will ultimately have to give way to adaptation or even retreat. Extremely complex defensive fortifications such as the flood barriers of London, Rotterdam and Venice are likely to remain isolated projects. For most other areas the development of modern risk management concepts would be more logical.